There has already been proposed a wire cut electro-discharge machine tool using an electrode wire of a wire shape or strip shape, which is provided with, for times when the electrode wire breaks during the machining, a clamp mechanism for clamping an electrode wire portion on a feeding side and an electrode wire feed mechanism for moving the electrode wire so that the end of electrode wire portion on the feed side coincides with a preset position and which thus enables automation of the insertion of the electrode wire portion of the feed side to the work (See Japanese Unexamined Patent Publication No. 56-3146). This known wire cut electrodischarge machine tool confirms the clamping of the disconnected end of the electrode wire by the above clamping mechanism, then refeeds the said wire disconnected end toward the electrode wire inserting hole of the work, then pulls back the electrode wire disconnected end projecting from the wire inserting hole of the work, holding it by a motor-driven electrode drawing mechanism, lays the electrode wire between upper and lower heads provided above and below the electrodischarge machining region, and thus prepares for restart of the wire cut electrodischarge machining. This known wire cut electrodischarge machine tool controls the wire cut electrodischarge machining process and the aforementioned machining restart preparation process at the time of electrode wire disconnection by means of a CNC device (computer numeral control device) using a microcomputer.
However, in a wire cut electrodischarge machine tool due to the electrodischarge action between the electrode wire and workpiece, or the tilted running of the electrode wire during taper hole machining, when the electrode wire breaks, the disconnected portion tends to be curved or rough surfaced. Therefore, often it is impossible to correctly refeed the disconnected electrode wire end through the wire inserting hole of the workpiece without some repair thereof.
Therefore, it has been considered necessary to use the afore-mentioned CNC device to automatically join the wire and prepare for restart of machining by eliminating curved portions of the electrode wire disconnected portion and roughed surface portions and further by preventing the removed portion from remaining on the work.